This adds support for optionally a providing a foreign Wayland surface
to a ClutterStage before it is first show. Setting a foreign surface
prevents Cogl from allocating a surface and shell surface for the stage
automatically.
v2: add CLUTTER_AVAILABLE_IN_1_16 annotation and API reference docs
(review from Emmanuele Bassi)
v3: set a boolean to indicate that this stage is using a foreign surface
(Rob Bradford)
https://bugzilla.gnome.org/show_bug.cgi?id=699578
